The present invention relates to an image forming apparatus which forms an image by an electrophotographic method, and more particularly to an improvement of a fixing device of the image forming apparatus.
(1) A technology for energy saving is one of the subjects of development in the technical field relating to the image forming apparatus which forms an image on a recording material by the electrophotographic method. The power consumption of the electrophotographic image forming apparatus depends upon the power consumption of the fixing device so largely that the energy saving in the fixing device represents the saving of power consumption in the image forming apparatus, and accordingly, the development of the technology for suppressing the energy consumption in the fixing device is the aforementioned subject of development.
Concerning the electric power which is consumed in the fixing device, the energy consumption under a stand-by condition is overwhelmingly greater than the energy consumption under an image forming condition. Accordingly, there is paid much attention for suppressing the energy consumption of the fixing device in the stand-by condition, that is, there is paid much attention to the development of the fixing device, which is kept under the condition that the power supply is not given to a heat source of the fixing device, or the condition that lower electric power is given even if power is given, and which can rise to the condition being capable of fixing within a short time, when a starting button of an image formation is operated, or when an instruction for an image formation is given from the outside.
A belt having small heat capacity is influential for a heating member of the fixing device having a short rise time mentioned above, and hitherto, there have been a great number of patent applications concerning the fixing device in which the belt is used for the heating member.
Further, it is performed that temperature of the heating member is raised to the temperature capable of fixing, while the heating member is released from a pressure applying member. Since the aforementioned rise-up of the heating member prevents heat of the heating member from traveling to the pressure applying member, the heat capacity of a heat receiving system becomes so small that the heating member can rise to be the temperature level capable of fixing within a short time.
(2) Generally, the fixing device is provided with the heating member and the pressure applying member. The fixing device fixes the toner image on the recording member by heat and pressure, by making both of the heating member and the pressure applying member to come into contact with each other by the prescribed pressure, and making a recording member having an unfixed toner image to pass between the heating member and the pressure applying member.
In the conventional fixing device, the pressure applying member is provided under the condition where the pressure applying member is brought into contact with the unmovable heating member, or is released from the unmovable heating member, and the pressure applying member is pressed to the heating member by the movement from the released position that is not under the acting condition, when the image formation is performed. Further, when releasing the pressure applying member from the heating member, the pressure applying member has been moved against urging of an urging means.
(1) It has become clear that if there is a difference between the surface speed of the heating member and that of the pressure applying member, when the pressure applying member is brought into contact with the heating member, the difference causes stress which gives undesirable influence upon these members. That is, it has become clear that the surface of the heating member or the pressure applying member changes in formation or changes in quality. Especially, when one having a rubber surface with lower hardness on the surface or a belt is used as the heating member, these changing are clear, and off-set occurs or the belt is broken in an extreme case.
The object of the present invention is to solve the above-mentioned problem of the fixing device wherein the pressure applying member is kept to be released from the heating member during the stand-by condition, and the pressure applying member comes in contact with the heating member when the image is formed, and further, to provide a fixing device of an energy saving type which can keep good fixing performance for a long time, still further, to provide an image forming apparatus having therein the above-mentioned fixing device.
(2) Under the construction that the pressure applying member is brought into pressure contact with the heating member by the urging means, the pressure applying member is brought into contact with the heating member by the prescribed pressure when pressure is applied. Therefore, it is necessary to apply releasing power which is stronger than pressuring power on the pressure applying member, when the pressure applying member is released from the heating member, which means that great power is necessary for releasing the pressure contact. A motor is used generally as a driving means which performs pressure contact/releasing of pressure contact of the pressure applying member, however, the motor having large power is necessary, resulting in problems that the electric power consumption is large and the cost is high.
Another object of the invention is to solve the above-mentioned problems of a mechanism which performs pressure contact/releasing of the pressure applying member onto the heating member, and to provide a fixing device of a type of low energy consumption and low cost, and to provide an image forming apparatus having therein the above-mentioned fixing device.